1. Field of the Invention
The present invention relates to a cast-coated paper. More particularly, the present invention relates to a process for producing cast-coated papers that have improved gloss characteristics not only before printing but also after printing, that have high surface strength, and that are free from surface imperfections such as pinholes and uneven adhesion between the base paper and the cast-coated layer.
2. Description of the Prior Art
Because of advantages such as high surface gloss and smoothness, cast-coated papers enable highly reliable so they are suitable for precise and high-grade printing operations and are extensively used in artistic printed matter, high-quality catalogs, the front covers of magazines, labels, wrapping papers, etc.
Cast-coated papers are conventionally produced by a process that comprises applying an aqueous pigment coating (i.e., a composition that contains a pigments and an adhesive as its main components) onto the surface of a base paper to form a pigment coating layer, which is is then pressed and dried in contact with a heated, highly polished metal drum. While this process is practiced in several ways, they can be roughly divided into three types, a wet (direct) method, a gelation (coagulation) method and a rewet (indirect) method in accordance with the manner in which the pigment coating layer is pressed and dried in contact with the heated, highly polished metal drum to get the cast-coated paper with a glossy surface. In the wet method, the pigment coating layer is pressed and dried in contact with the heated metal drum while it is still in a wet and plasticized state. In the gelation method, the pigment coating layer is passed through a coagulating bath so that it gels and becomes plasticized before it is pressed and dried in contact with the heated metal drum. In the rewet method, the pigment coating layer is first dried, then optionally supercalendered, and thereafter rewetted with a wetting solution to be rendered plasticized so that it can be pressed and dried in contact with the heated metal drum to give a glossy surface.
The three basic processes for producing cast-coated papers share the common feature that the pigment coating layer, while it is in a plasticized state, is pressed and dried in contact with a heated, highly polished drum. Hence, the water in the pigment coating layer partly evaporates through the base paper and partly through the pigment coating layer toward the drum surface. If the rate of water evaporation exceeds a certain level, pinholes will develop in the surface of the cast-coated layer and the resulting cast-coated paper is no longer suitable for use in printed matter. In other words, the rate of production of cast-coated papers is reduced. This phenomenon is particularly noticeable in the wet process which involves fairly extensive water evaporation, so that wet method has been increasingly supplanted by the rewet process in commercial operations for high-speed production of cast-coated papers.
The rewet process, however, has the disadvantage that the pigment coating layer is not as highly plasticized as in the wet and gelation processes. Hence, the recipe of aqueous pigment coatings for providing cast-coated layers, the selection of a rewetting solution, and the selection of operating conditions such as the working speed, the temperature of a highly polished drum and the pressure at which the pigment coating layer is brought into contact with the drum are important in the practice of the rewet method and if these factors are not properly selected, the surface of the cast-coated layer will develop various imperfections such as pinholes and uneven adhesion to the base paper.
The gelation process in which the pigment coating layer is made to gel by treatment in a coagulating bath enables casting operations to be carried out at fairly high temperatures. On the other hand, the gelation process usually involves no drying step, so the drying load on the surface of the casting drum is higher than in the rewet process. Further, the coagulation step which is performed just prior to the casting operation is incapable of providing satisfactory gelation, and the coagulant used has the potential to reduce the strength and gloss of the cast-coated paper.